Magnetite particles as one kind of magnetic iron oxide particles are a typical black pigment, and have been longtime used as a colorant for paints, printing inks, cosmetics, rubbers and resin compositions, or the like. Also, the magnetite particles have been used as a friction material such as a disk brake pad, a brake lining, etc., for braking automobiles, rail road vehicles or various industrial equipments, and further used as magnetic iron oxide particles for a resin carrier in an electrophotographic developer because of a high magnetization value thereof.
In particular, in the case where the magnetic iron oxide particles are used for a resin carrier in an electrophotographic developer, there tends to arise such a problem that the magnetic iron oxide particles are fallen off from a surface of the carrier upon development and scattered around, which tends to result in deteriorated developability of the electrophotographic developer. For this reason, it has been required that the magnetic iron oxide particles are prevented from being fallen off from the surface of the carrier.
Therefore, in order to satisfy the above requirements for the resin carrier, it has been strongly required that the magnetic iron oxide particles are enhanced in bonding property to a resin.
In order to enhance a bonding property between the magnetic iron oxide particles and a resin, it has been attempted to use those magnetic iron oxide particles having protrusions or those magnetic iron oxide particles subjected to a lipophilic treatment.
For example, in Patent Document 1, there are described the magnetic iron oxide particles each including a base particle portion having a spherical shape and angular protrusions formed on a surface thereof.
Also, in Patent Document 2, there are described the magnetic metal compound particles whose surface is subjected to a lipophilic treatment.
In addition, in Patent Document 3, there are described the magnetic iron oxide particles whose aggregated particles are well controlled in particle diameter.
As is well known in the art, in electrophotographic methods, there has been in general employed the following developing method. That is, a photosensitive member formed of a photoconductive material such as selenium, OPC (organic photoconductor), a-Si or the like is used to form an electrostatic latent image thereon by various means. Then, by using a magnetic brush developing method or the like, a toner electrically charged into a polarity reverse to that of the latent image is attached thereonto by the electrostatic force to develop the latent image.
In the above developing step, there is used a two-component system developer comprising a toner and a carrier. The carrying particles called a magnetic carrier act for imparting an appropriate positive or negative electric charge amount to the toner by frictional electrification, and delivering the toner into a developing zone near the surface of the photosensitive member on which the latent image is formed, through a developing sleeve in which magnets are accommodated, using a magnetic force thereof.
The above electrophotography has been extensively applied to copying machines or printers. In recent years, these apparatuses have been rapidly functionalized, i.e., digitalized and formed into a composite structure and, at the same time, have been required to have a high image quality and a high copying or printing speed. With these requirements, there is an increasing demand for developers used therein which are improved in various properties.
In particular, with the increase in image quality and copying or printing speed of these electrophotographic apparatuses, it has been demanded that a carrier used in a developer for developing color images further has a high reliability. Thus, it is required that the carrier used in the developer has a long service life such that various properties of the carrier such as an electric resistance thereof can be stably maintained over a long period of time.
In a developing device, the carrier is always subjected to repeated frictional contact with a toner, other carrier particles or an inside portion of the developing device, and therefore suffers from a shear force. As a result, there tend to be caused problems such as change in electric resistance value of the resin-coated carrier and formation of spent toner.
The iron powder carrier and ferrite carrier are usually used in the form of resin-coated particles obtained by coating a surface thereof with a resin. However, since the iron powder carrier has a true specific gravity as large as 7 to 8 g/cm3 whereas the ferrite carrier has a true specific gravity as large as 4.5 to 5.5 g/cm3, a large driving force is required for stirring these carriers in the developing device, resulting in significant mechanical damage to the device, occurrence of spent toner as well as change in electric resistance of the carrier itself and facilitated damage to the photosensitive member.
The carrier of a magnetic material-dispersed type comprising spherical composite particles formed from magnetic fine particles and a phenol resin as described in Japanese Patent Application Laid-Open (KOKAI) No. 2-220068 has a true specific gravity as small as 3 to 4 g/cm3 as compared to the iron powder carrier or ferrite carrier, so that the energy upon impingement between the carrier and the toner becomes small. Therefore, it is advantageous to attain a durable retention property of an electric resistance value of the resin-coated carrier and suppress formation of spent toner.
However, with the increase in functionality, image quality and copying or printing speed of recent copying machines or printers, there tends to arise such a problem that when a shear force is applied onto the carrier in the developing device, the carrier suffers from change in electric resistance value to cause deterioration in image quality, adhesion between the carrier particles or the like.
In particular, in recent years, with the progress of maintenance-free systems, a developer used therein is often required to have a durability over a long period of time which may correspond to almost a whole service life of the maintenance-free machines. Therefore, it is strongly required to provide a carrier having an excellent durability.
Conventionally, with respect to the magnetic material-dispersed type carrier for electrophotographic developers, there are known the techniques which have noticed surface conditions of particles, e.g., the technique of forming fine irregularities owing to plate-shaped metal oxide particles being present on a surface of respective spherical composite particles to control surface conditions of a magnetic carrier according to a fluidity rate and an average particle diameter thereof (Patent Document 5); the technique of forming fine irregularities on a surface of respective particles obtained by dispersing ferromagnetic iron oxide particles in a phenol resin to control surface conditions of a magnetic carrier according to a ten-point mean roughness thereof (Patent Document 6); or the like. Also, as the techniques which have noticed aggregating conditions of particles, there are known the technique concerning magnetic iron oxide particles which are controlled in particle diameter of aggregated particles thereof (Patent Document 3) and the like.